1. Field of the Invention
The object of the invention is a security feature and a corresponding proof process for security documents, which process is based on the piezo effect.
2. Description of the Related Art
Hitherto there has been merely known the practice of applying the security features to security documents and documents of value in the form of particles, these particles having been formed as electro-luminescence elements. Such electro-luminescence elements react in an electromagnetic alternating field, as they emit a given light. A precondition for this security feature, to be sure, is the applying of an external electromagnetic field, which is associated with relatively high expenditures. Furthermore, the application range of these security and value documents provided with EL particles is restricted to the condition that only a light of a given wavelength is emitted.
Underlying the invention, therefore, is the problem of further developing security and value documents of the type mentioned at the outset in such manner that a further security feature can be added. The use purpose of such a document is therewith to be extended. Besides the optical detection of security features there is further to be made possible a detection in another frequency range. An additional feature of the present invention lies in that a process is described for the detection of this security characteristic.
The solution of the first problem occurs essentially through the feature that the value and security document is equipped with piezo-electric properties.
The solution of the second problem occurs essentially by the means that the value and security document is exposed to the signals, in which the piezoelectric properties are utilized. The signals given off from the piezo elements are thereupon detected and evaluated.
In a preferred embodiment of the invention it is provided that the piezoelectric properties of this value and security documents were introduced onto or into the document in the form of piezo material.
If value and security documents or other relevant products are equipped with piezoelectric security features, then these can be mechanically excited (for example by impacts or acoustically) or optionally electrically, and their answers can be detected and evaluated. From the system answer of the value and security document it is possible to infer the materials used and their geometrical dimensions. A comparison of test piece and reference sample makes possible a distinguishing of genuine and forged value papers or products.
The equipping of relevant products with piezoelectric security features can occur in the following manners:
piezoelectric micron particles are admixed to the printing inks or other application layers,
piezoelectric micron particles are admixed to the base material of the security product (for example paper, foils, glass) during the production,
metallized piezo-polymer foils in the micron range are applied as intermediate product and/or layer in or onto the value paper/product (gluing, laminating, laser welding etc.), or
piezo ceramic platelets are laminated in with thicknesses of much less than 100 microns.
As micron particles there are designated elements the geometric dimensions of which lie in the range of microns.
Excited with impulses, the piezo elements react with vibrations which again are electrically or mechanically (acoustically) transferred and can be suitably evaluated. Especially the natural frequencies of these vibrations are a highly significant characteristic for types of materials used and their geometric dimensions. As additional information parameters there can be evaluated the signal damping (FFT) of a transmitted signal.
In the invention therefore both the piezo effect and also the reciprocal piezo effect is drawn upon for the evaluation.
Thus there is obtained with the frequency spectrum according to reception signal scanning and FFT, a sort of xe2x80x9cFingerprintxe2x80x9d of the value paper or product.
In a preferred embodiment of the present invention there is provided that in such a value and security document there are embedded piezo-electrically active particles. Here it is not a matter of the grain size and the type and the arrangement of these particles in detail. They can be either uniformly embedded into the material of the value and security document, in which case, however, they could be heaped up in certain places of the value and security document.
In another embodiment they can be applied to the surface of the value and security document, (and be embedded into the document on the surface, or they can additionally be covered by means of a covering layer, a lacquering, a plastic covering or a lamination foil.
In a further embodiment of the invention it can be provided that instead of using individual particle-form piezo materials, a piezo effect may also be produced in a polymer foil.
As an example for such a polymer foil there is to be mentioned a PVDF foil.
Instead of the use of individual piezo particles that are applied in uniform or non-uniform form to or on a value or security document it is possible in another embodiment to sinter such elements, especially in crystal structure, in order to create from them a larger piezo element. Also such a larger piezo element can be embeddedxe2x80x94as stated earlierxe2x80x94either on the surface or at the surface of the value and security document or also embedded in the surface.
What is important in the invention is that to the first mentioned security feature known per se, i.e. the emitting of a certain light spectrum, there is now allocated a further security feature which can act standing alone or in connection with other security features. In the corresponding excitation of the piezo material mentioned, therefore, a vibration answer is generated to a corresponding mechanical or acoustical excitation of the piezo material.
On mechanical loading of certain crystals, such as, for example, quartz, tourmaline, Seignette salt in given directions to the crystal axes there occur, namely, electrical displacements, consequently free surface charges, which are proportional to the generating force. This piezoelectric effect is suited as the basis for the security feature described here. The preferably used quartz crystallizes in hexagonal prisms. Correspondingly to its atomic structure it has 3 polar electric axes, 3 mechanical neutral axes and one optical axis standing perpendicularly to these. From the crystal then, parallel plates are cut out, which are suitable as feelers.
If these are pressed in X- or Y-direction, then there arises on the X-Z surfaces the charging of the piezo module.
The invention is not restricted, however, to the use of mono-crystalline crystals, butxe2x80x94as indicated earlierxe2x80x94crystal agglomerates can be used, or also sintered crystals.
Instead of such crystals ceramic bodies can also be used, which are likewise known to have a piezoelectric effect.
The excitation of such a piezo material occurs preferably with an acoustic or electric impulse. Likewise the excitation can occur with a signal source in which all the frequencies are present, which, therefore generates a wide-band, white noise.
Instead of these excitation mechanisms the excitation can also occur by means of a laser impulse.
The vibration answer of the piezo material occurs in the sense of a natural frequency of this material. If, therefore, excitation is performed with a single impulse, this piezo material is excited up to free-running vibration and it vibrates over a certain impulse duration with a natural frequency which can be detected and evaluated outside of the security and value document. The frequency of the vibration answer lies here in the kilohertz up into the gigahertz range and can be picked up and evaluated with corresponding measurement value receivers.
Preferably, therefore, for the vibration stimulation a single impulse generator is used, which, therefore, mechanically delivers a single impulse onto the value and security document to be tested and with corresponding measurement value receiver technique, then, the vibration answer is detected and evaluated.
Here it is important that always the total system of the value and security document is checked. If, therefore, for example adhesive additives, material denudation fragments or other (also local) attacks were made on the document, then therewith (also locally limitedly) the system-answer of the total system is displaced.
In another embodiment a continuous excitation can occur by means of a white noise which is delivered to the value and security document over a corresponding vibration generator.
Likewise the excitation of the value and security document can occur in feedback operation, i.e. a certain excitation frequency is used; the answer frequency is then investigated and fed back onto the excitation frequency, in order in this way to obtain a vibration excitation and to excite the piezo material in the range of its resonance frequency.
The type of resonance frequency is then an image of the vibration property of the piezo material, and is therewith characteristic as a fingerprint for the security feature which is accommodated in the security document. It is a matter here of a peak pattern which is generated not at a single characteristic frequency, but in a certain frequency band.
The resonance frequency is dependent on the geometry of the individual piezo grain. The effects described earlier obviously serve in a like manner for the piezo electric foil already described, the PVDF foil which is excited in the same manner and that generates a corresponding vibration answer.